Arrest warrants issued after men rape teenage girl in Rio de Janeiro and put footage online.

Brazilian police say they have identified four of 33 suspects involved in the gang rape of a 16-year-old girl in Rio de Janeiro that intensified public calls for action against the endemic problem.

The four arrest warrants were issued on Friday, days after the assault came to light when several men joked about the attack online on Wednesday, posting graphic photos and videos of the unconscious, naked teen on Twitter.

"It's absurd that in the 21st century we have to live with barbarous crimes like this," Brazil's acting President Michel Temer said in a statement, promising to create a federal police force unit tasked with tackling crimes against women.

At a news conference, police said the girl said she was raped by 33 men on Saturday, and she regained consciousness the following day.

The girl's 19-year-old boyfriend was one of the men being sought, but police said they did not know whether he may have been one of the attackers.

Police said the men were armed, though it was not clear if the weapons were used to intimidate the victim during the attack.

In another case, a group of at least five men allegedly drugged and raped a 17-year-old girl in the northeastern state of Piaui.

"Apart from the fact that these are young women, these barbaric cases are similar in that the teenagers were lured by their attackers in premeditated plots," said Nadine Gasman, the spokeswoman for UN Women in Brazil, on Thursday.

"They were violently attacked in a context of illegal drug use."

The posting of the video by the perpetrators sparked a wave of revulsion among Brazilians on social media.

The Brazilian non-government group, Public Safety Forum, said in a study that Brazilian police recorded a sexual assault every 11 minutes in the country in 2014.

An estimated half-a-million such assaults take place each year overall.